Conventionally, in order to impart oil resistance to paper, an approach has been taken to make the critical surface tension of a treated surface smaller than the surface tension of an oily substance. Chemicals with such function are called oil-resistant agents, and oil-resistant paper treated with a fluorine-based oil-resistant agent has been mainly used.
For example, as Patent Document 1 presents a fluorine-based oil-resistant agent as a novel oil-resistant agent, those in which a fluorine-based compound such as acrylate or phosphoric ester of perfluorocarbon is used have been mainly used because they are inexpensive and effective.
In the case of oil-resistant paper using a fluorine-based oil-resistant agent, no coating needs to be formed because the oil-resistant agent itself has excellent oil repellency and strong water repellency, and thus the oil-resistant paper is able to have a lowered resistance to air permeability.
Recently, however, it has been revealed that when fried food is wrapped with such oil-resistant paper using an oil-resistant agent of a fluorine-based compound and heated up in an electronic oven at a high temperature of 100° C. or more, harmful gas (fluoroalcohol gas, hydrogen fluoride, etc.) that can be accumulated in the human body is generated, and thus use of the fluorine-based oil-resistant agent has posed a serious problem. It is also pointed out that even without heating by the electronic oven, similar gas may be generated when such paper is used for packaging a food material having a temperature of 100° C. or more.
In addition, fluorine-based organic compounds have extremely poor biodegradability and there is a global concern for pollution due to these substances. As above-described, due to such danger to human health and impact on global environment, use of fluorine-based compounds now poses serious social problems.
As oil-resistant paper in which no fluorine-based compound is used, Patent Document 2 proposes an oil-resistant container prepared by applying a silicone resin and an adhesive for heat sealing to a barrier layer containing crosslinked polyvinyl alcohol and/or starch and a water resistant additive as main components and then forming the material into a container. This oil-resistant container did not always have satisfactory oil resistance and had a problem that the cost of the container was high because silicone resin was expensive.
Further, Patent Document 3 proposes an oil-resistant paper prepared by applying a coating layer containing nonionic or cationic polyvinyl alcohol and a coating layer containing a fluorine-based oil-resistant agent in that order. However, the coating layers containing polyvinyl alcohol and starch employed in this document have a role of preventing permeation of the fluorine-based oil-resistant agent into paper, which is quite different from the subject matter of the present invention.
Patent Document 4, Patent Document 5 and Patent Document 6 propose oil-resistant paper using acrylic emulsion as an oil-resistant agent. However, these kinds of oil-resistant paper require a thick acrylic resin coating for satisfying desired properties, and this resulted in an extremely high resistance to air permeability and properties as a food packaging material were lost. When a food packaging material has a high resistance to air permeability and food is heated or kept warm within the packaging material, the inside of the package is filled with vapor generated from food, and there may be a case that food is moistened with condensed dew and quality and taste of the food is remarkably degraded. In addition, when the food is heated again in an electronic oven still being wrapped with the packaging material, rapidly generated vapor cannot escape to the outside and the package may be broken. Moreover, in order to form a coating having sufficient oil resistance, a large amount of coating must be applied and this has resulted in a problem of high costs of packaging materials.
On the other hand, in order to ensure high oil resistance, lamination of film has been generally practiced. However, when film is laminated, resistance to air permeability becomes extremely high, and the resulting packaging material was defective as described above.
To prevent the resistance to air permeability becoming extremely high, Patent Document 7 proposes an air-permeable oil-resistant sheet comprising a substrate such as paper having pores and a thermoplastic film having pores similar to that of the substrate laminated on at least one side of the substrate. It is also proposed to form a laminate of non-woven fabric and paper. However, such sheets cannot fully prevent edible oil from bleeding to the outside, and excellent oil resistance has not been achieved.
Patent Document 8 proposes oil-resistant paper using a hydrophobically modified starch, i.e. hydorophobized starch, but the oil resistance of this paper was not always satisfactory. To achieve sufficient oil resistance by using only the hydrophobized starch, an enormous amount of hydrophobic starch must be applied, and this is impractical in terms of the cost. In addition, increase in resistance to air permeability due to the increased coating amount was also a problem. Further, in the case of using the hydrophobized starch alone, starch is easily soluble in water, and when the material is used as a food packaging material, starch is dissolved in hot water and stains the food.
Patent Document 9 proposes oil-resistant paper coated with an oil-resistant agent in which polyvinyl alcohol is used or polyvinyl alcohol and crosslinking agent are used together. This invention accomplishes high oil resistance in small coating amounts, but because polyvinyl alcohol is used, resistance to air permeability was not satisfactory although it could be kept lower as compared to those of other oil-resistant agents containing no fluorine. Further, coating using a size press involved a problem of contamination of dryers.
In addition, a fatty acid sizing agent for internal addition is known to improve oil resistance, but this oil resistance means just to prevent strike through of ink at the time of offset printing, and is remarkably lower than the oil resistance required in food packaging materials as defined in the present invention. Thus, use of the fatty acid sizing agent for oil-resistant paper has not been studied. Further, the mechanism of producing oil resistance by using the fatty acid sizing agent is quite different from that of the present invention. Moreover, the fatty acid sizing agent is generally used as an internal additive to improve Stockigt sizing degree, and hardly added to a coating layer, and yet, addition thereof to a coating solution for improving oil resistance has not been studied at all.
As described above, prior arts have not produced oil-resistant paper suitable as food packaging materials simultaneously satisfying oil resistance, resistance to air permeability and productivity.    [Patent Document 1]: Japanese Patent Laid Open No. 12-026601    [Patent Document 2]: Japanese Patent Publication No. 6-2373    [Patent Document 3]: Japanese Patent Laid Open No. 8-209590    [Patent Document 4]: Japanese Patent Laid Open No. 9-3795    [Patent Document 5]: Japanese Patent Laid Open No. 9-111693    [Patent Document 6]: Japanese Patent Laid Open No. 2001-303475    [Patent Document 7]: Japanese Patent Laid Open No. 11-021800    [Patent Document 8]: Japanese Patent Laid Open No. 2002-69889    [Patent Document 9]: Japanese Patent Laid Open No. 2004-68180